The present invention relates to an optical scanning system for electrostatic photography.
Optical scanning systems for electrostatic photography are known in the art in which at least one of a document platen, a scanning ahead and a photoconductive member is moved to accomplish scanning of a document on the platen and imaging of the photoconductive member. The imaging system is provided with a slit which serves as an exposure aperture.
It is desirable to be able to vary the intensity of the light image incident on the photoconductive member in accordance with the characteristics of the particular original document for electrostatic reproduction. Prior art systems generally vary the width of the slit to accomplish this result. However, various drawbacks are inherent in such a method. Due to the configuration of light sources and optical elements of the scanning system, the intensity of the image along the length of the slit becomes non-uniform at various widths of the slit, with the pattern of non-uniformity changing in dependence on the slit width. The slit width must be small in order to produce a sharp image. An imaging system in which the slit width is reduced below a maximum value to control the image intensity is inherently wasteful of the light illuminating the document. Generally, there is an optimum value of slit width which provides an image of maximum quality, and any substantial change from this value causes a noticeable degradation of the image.
Another drawback in known imaging systems is that the scanning head is moved in one direction to effect scanning and then returned to its original position. The return movement of the scanning is therefore non-productive constituting inherent inefficiency in the system which necessitates excessives copying time.